fix(model): users 唯一索引改为 (shop_id, username) 复合索引

原来 uniqueIndex:uk_shop_username 只在 Username 字段上，导致用户名全局唯一。
S002/S003 的 admin 用户与 S001 冲突，INSERT 被静默跳过，无法登录。

将 User 的 TenantBase 改为直接定义 ShopID，使其参与复合唯一索引。
需要 Reset DB 使新索引生效。

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/cmd/seed/main.go

@@ -132,21 +132,21 @@ func main() {
 	hash := mustHash("password123")
 	admin := upsert(db, &model.User{}, "shop_id = ? AND username = ?", shop.ID, "admin", func() any {
 		return &model.User{
-			TenantBase: model.TenantBase{ShopID: shop.ID},
+			ShopID: shop.ID,
 			Username: "admin", PasswordHash: hash, RealName: "张三（管理员）",
 			Phone: "13800000001", Role: "admin", IsActive: true,
 		}
 	}).(model.User)
 	operator := upsert(db, &model.User{}, "shop_id = ? AND username = ?", shop.ID, "operator", func() any {
 		return &model.User{
-			TenantBase: model.TenantBase{ShopID: shop.ID},
+			ShopID: shop.ID,
 			Username: "operator", PasswordHash: hash, RealName: "李四（操作员）",
 			Phone: "13800000002", Role: "operator", IsActive: true,
 		}
 	}).(model.User)
 	upsert(db, &model.User{}, "shop_id = ? AND username = ?", shop.ID, "test", func() any {
 		return &model.User{
-			TenantBase: model.TenantBase{ShopID: shop.ID},
+			ShopID: shop.ID,
 			Username: "test", PasswordHash: hash, RealName: "王五（只读）",
 			Phone: "13800000003", Role: "readonly", IsActive: true,
 		}